Discharge tube



Bec. 24, 1,11%32 M. KUIPERS 2,225,645

DIS CHARGE TUBE Filed Dec. 1'?, 1938 Patented Dec. 24, 1940 UNITED STATES PATENT GFFICE DISCHARGE TUBE trustee Application December 17, 1938, Serial No. 246,441 In Germany January 3, 1938 4 Claims.

My invention relates to ionic discharge tubes for rectifying high alternating voltages which operate with an arc-like discharge and have an incandescible cathode, preferably of the activated type.

The ionizable filling of these tubes may consist of a gas, a vapor or a mixture thereof, and the term gaseous lling is to be understood to cover any of these possibilities.

To increase the blocking power of such tubes at low or medium voltages, for instance up to 300 Volts, it has been proposed to use a substantially cylindrical anode, and, coaxially thereto, a similarly shaped metal screen laterally surrounding the cathode and having one or more apertures located in front thereof, said screen extending in a transverse direction at least to such a multiple of the distance between the mutually parallel and plane surfaces of the anode and the screen that a substantially homogeneous electrostatical field is formed in the intervening space.

Moreover it has been proposed to arrange said screen around the cathode on all sides and to adapt the diameter of the cylindrical outer surfaces of the anode and the screen to the inner surface of the glass container, so as to preclude any passage of the operative discharge to the supply conductors and the seals of said electrodes.

When utilizing tubes of such construction to rectify very high alternating voltages, for instance of thousands of volts, without employing the so-called anode arms, it was necessary to utilize expensive and complicated screen con-` structions for the purpose of completely preventing the production of undesired discharge phenomena, for instance back-discharges. Especially, it was necessary to use elaborate screening of the supply conductors, whereas the close manner in which the cathode was surrounded by its screen, counteracted effective ionization as required to overcome the cathodic space charge, in connection with the very low pressures of the gasor vapor filling of the tube that were inevitable in view of the high voltages that had to be blocked in the non-conductive direction. The occurring restriction of the ionizable space around the cathode gave rise to an excessive voltage drop in the conductive direction, of the order of about 20-30 volts, which in turn reduced the load capacity of the tube, or in the case of equal power unduly increased the size of the envelope of the tube. In addition, the forward ignition voltage also reached an excessive figure, as a result of the use of these screens. These (CL- Z50-27.5)

difliculties were due to the fact that-a substantially higher level of energy had to be reached to form suicient ions to overcome the cathodic space charge and thus to start an arc-like discharge.

The main object of my invention is to overcome the above diiculties and to provide an extremely simple screen construction.

Another object is to provide a tube with which it is possible to rectify an appreciable current at a high blocking voltage and in a comparatively small tube of simple construction, Without having to take into account a forward voltage drop which considerably exceeds the values usually occurring in low-voltage tubes. To this end, according to the invention, several measures are combined by which it is possible to obtain a surprisingly favourable compromise between the conicting requirements discussed hereabove.

According to the invention, the following measures are taken in connection with tubes of the kind described hereabove.

(1)l The preferably cylindrical cathode is arranged immediately behind and parallel to the plane front surface of the cathode screen, so that its part most adjacent to the anode is screened therefrom in a direction normal to its frontal surface, and that the electrostatical influence of the anode through the apertures toward the cathode is such that, in the case of a high vacuum and considering the screen as the control grid of a triode, an amplification factor of 10 or less would result.

(2) The cathode screen is open in the direction opposite to that of the anode, and the cylindrical part of the screen extends in said direction to approximately the same distance from the anode as the part of the cathode that is most remote therefrom, and it is closely surrounded by the envelope at least at its edge adjacent to the anode.

(3) The parallel surfaces extend in a transverse direction at least to the quadruple of their mutual distance d, which, together with the operating pressure p of the gas or vapor-filling, is so chosen that the value of the product pd lies between the limits of 0.15 103 Inm. cm. and 15 10-3 mm. cm.

The measures mentioned hereabove are intended to realize, on one side, a most favorable p d value lying a good deal to the left of the minimum of Paschens curve (ignition Voltage as a function ofV pd), on the other side also to prevent any back ignition resulting from an excess of unhindered linesof force extending to rear of the cathode, all the same avoiding the use of such of positive ions accelerated by the electrostatic field of the anode, without any undue restriction or lengthening of the discharge path on its Dassage through the screen.

The very low forward voltage drop which does not exceed values usual in ordinary low voltage tubes, permits ofr a considerable reduction of the physical dimensions of the tube without any undesired temperature rise, so that the tube presents the outline of a low voltage tube and behaves like one in the conductive direction but nevertheless shows most surprising blocking qualities. For instance the tube represented in the drawing has the dimensions of a usual radio tube.

The invention allows one to cope without anyv diiculty with blocking voltages of the order of 15,000 volts with a simple screen even if i-t is not arranged with very precise clearance from the wall of the tube over a material length, as known tubes often do, and without the use of insulating tubes surrounding the supply conductors of the electrodes.

This favorable functioning of the tube may be particularly ascribed to the fact that, the electrostatic field between the anode and the screen being very homogeneous during the non-conducting phase, it is possible to protect the cathode surface against ionic bombardment without having recourse to such elaborate screening that difficulties would arise in the conductive direction, always provided that certain minimum screening be complied with, -as well as the requirements formulated hereabove with respect to the pd value and to the spatial conditions behind the cathode.

In order that the invention may be clearly understood and readily carried into effect, I shall describe the same in more detail with reference to the accompanying drawing, in which,

Figure 1 is a partly sectioned side view to scale of a rectifier tube according to the invention,

Fig. 2 is a sectioned View along line 2-2 of Fig. 1,

Fig. 3 is a top View of a screen.

The rectifier tube shown in the drawing comf prises an envelope I of vitreous material forming a press I5. Mounted on press I5 and connected with one end to leads I6 are two leadsupports 4 who-se free ends are connected toV an incandescible cathode 2 arranged perpendicular to the axis I8-I8 of the tube.

Cathode 2 is formed of a helical wire, for instance, of tungsten, upon which is helically wound a helix of thin wire, for instance tungsten, and is provided in the usual manner with a surface layer of a material of high electron emissivity, such as barium oxide. When the pressure of the filling within the tube has a low value, for instance of several microns, it is advisable to so interspace the turns of this helix that the spaces between turns are about equal to the diameter of lthecompleted helical wire. This brings about a considerable reduction of the voltage in the conductive direction, and therefore an appreciable increase in the life of the cathode or tube. A cathode of such construction has been described in more detail in the U. S. application Ser. No. 5

16,486 to Reerink et al., led April 15, 1935, and the manner of spacing the turns has been described in detail in the U. S. application Ser. No. 149,234 to Reerink et al., led June 19, 1937, now Patent No. 2,17 0,034. l Hermetically sealed at the top of the domeshaped portion of envelope I is a metal rod 9 to the lower end of which is secured an anode 8 in 'the form of a cup-shaped member of suitable metal, such as molybdenum or nickel, having a 1 frontal surface I9 and circumferential cylindrical extension 20.

Arranged coaxially with envelope I and supported from press I5 by two metal rods 5 is a cupshaped screen 3 which partly surrounds cathode 2. Screen 3, which also may be of molybdenum or of nickel, has a frontal surface 2l provided at the central portion with a rectangular aperture 22 whose cross section substantially coincides with that of cathode 2 in a projection normal to said frontal surface (see Fig. 2). This meansthat the surface areas of both cross sections are substantially the same. In the middle of `aperture 22 and parallel to the axis of cathode 2 is a conductive strip- I0 secured to screen 3. However, as shown in Figure 3, two apertures 23 and 24, separated by a strip 25, may be provided in the screen 3. Thus, that portion of the cathode 2 which is closest to anode 8 is screened by conductive strip I0 or strip 25 in a straight line with re- -spect to the anode. Screen 3 is arranged immediately in front of the cathode -2 yand has a cylindrical extension portion II which is connected with a rounded edge to the flat frontal surface 2 I, and which extends in the direction away from the anode 8 so yas tooverlap cathode 2, as seen from the side.

A cathode potential, or a voltage which is negative with respect to this potential, can be applied to screen 3 through a supply conductor 'I connected thereto, with the result that the majority of the leakage lines of the electrostatic field existing between screen 3 and anode 8 will start from the extension portion II, and that there is little chance of a eld suflicient to ignite va discharge being set up on the back of the cathode. For the same reason it is desirable not to use too large a spacing between screen 3 and the envelope I, because otherwise too many unimpeded paths for lines of force between the anode and 5 the cathodewould be available. However, if portion I'I is too close to the wall of the envelope I, thel ignition of the tube inthe conductive direc-` tion is rendered unnecessarily dificult. In the geometric conditions shown and with 4a screen po-- tential lying in .the neighborhood 'of the cathode potential there exists between the anode and the cathode an amplification factor of about 10. This screen potential corresponds to the optimum conditions of operation of the tube. f

The screen 3v in Fig. 2 has a diameter of about 27.15 mms. and the rectangular aperture 22 has a width of about 9 mms. and a length of about 20 mms., and the strip has a width of about 3 mms., thereby forming two elongated apertures each having a width of about 3` mms. and a length ofl about 2.0 mms. The extension II (shown in Fig. 1) oi the screen 3 has a length of 7 mms. and the spacing d between the frontal surfaces I9 and 2-I isiabout5 mms.

The further dimensions of l the tube may be secured with the use of the scale shown on the right of the gures.

In the lower portion of the envelope is some liquid mercury I2 which furnishes the discharge chamber with saturated mercury vapor and which is provided at the coldest point of the tube. It is advisable to maintain this supply at about 35 C., which corresponds to a vapor pressure of 4 microns. Thus, if the spacing d is 0.5 cm. the very favorable pd-value of 2 10-3 mm. X cm. is obtained.

The construction according to the invention has the special advantage that with the abovedescribed tube it was unnecessary to surround any supply conductor or electrode supports by an insulating tube as Was otherwise necessary with a gaseous-filled high-voltage tube, even when provided with anode arms. This also affordsv an app-reciable simplification in the construction of the tube.

While I have described my invention with reference to a specific example, I do not wish to be limited thereto but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is:

1. An ionic discharge tube comprising an envelope, a substantially cylindrical anode closely surrounded by said envelope and having a plane frontal surface, an` incandescible cathode, an apertured substantially cylindrical metal screen coaxial with said anode and surrounding said cathode, said screen having a frontal surface parallel to said anode frontal surface and being spaced therefromv by a distance d, and a gaseousfilling within said envelope, the operating pressure p of which, together with said distance d, yields a pnl-value lying between 0.15 l03 and 15 103 mm. cm., said incandescible cathode being arranged immediately behind the apertures of said screen so as to obtain a material electrostatical influence of the anode through said apertures and with respect to said cathode but to be screened from the anode on its part nearest thereto and in a direction normal to its frontal surface, said cathode screen being open in the opposite direction.

2. An ionic discharge tube comprising an envelope, a substantially cylindrical anode closely surrounded by said envelope and having a plane frontal surface, an incandescible cathode, an apertured, substantially cylindrical metal screen coaxial with said anode and surrounding said cathode, said screen having a frontal surface parallel to said anode frontal surface and being spaced therefrom by a distance d, and a gaseous filling within said envelope the operating pressure p of which, together with said distance d, yields a p.dvalue lying between 0.15 103 and 15x103 mm. cm., said incandescible cathode being arranged immediately behind the apertures of said screen so as to obtain a material electrostatical influence of the anode through said apertures and with respect to said cathode but to be screened from the anode on its part nearest thereto, and in a direction normal to its frontal surface, said cathode screen being closely surrounded by said envelope at least at its edge adjacent thereto and being open in the opposite direction, the cylindrical part of said screen extending, in the same direction, about as far as the cathode.

3. An ionic discharge tube comprising an envelope, a substantially cylindrical. anode closely surrounded by said envelope and having a plane frontal surface, an incandescible cathode, an apertured and substantially cylindrical metal screen coaxial with said anode and surrounding said cathode, said screen having a frontal surface parallel` to said anode frontal surface, being spaced therefrom by a distance d and extending in a transverse direction at least the quadruple of d, and a gaseous-filling within said envelope the operating pressure 10` of which, together with said distance d, yields a p d-value lying between 0.15 103 and 15x10-3 mm. cm., said incandescible cathode being arranged immediately behind the apertures of said screen, so as to obtain an electrostatical influence of said anode through said apertures and with respect to said cathode equivalent to an amplification factor of not more than 10, but to be screened from the anode on its part nearest thereto, and in a direction normal to its frontal surface, said cathode screen being closely surrounded by said envelope at least at its edge adjacent thereto and being open in the opposite direction, the cylindrical part of said screen extending, in the same direction, about as far as the cathode.

4. An ionic discharge tube comprising an envelope, a substantially cylindrical anode closely surrounded by said envelope and having a plane frontal surface, a substantially cylindrical incandescible cathode, an apertured substantially cylindn'cal metal screen coaxial with said anode, said screen surrounding said cathode and having a frontal surface parallel to said anode frontal surface and being spaced therefrom by a distance d, the frontal surface of said screen being provided with two slits parallel to each other and to the axis of the cathode, and a gaseous filling Within said envelope the operating pressure p of which, together with said distance d, yields a pri-value lying between 0.15 103 and 15 103 mm. cm., said substantially cylindrical incandescible cathode being arranged parallel to the frontal surface o-f said screen and immediately behind the slits therein, the outline of said cathode substantially coinciding, in normal projection onto the frontal screen surface, with the rectangular external outlines of said slits, their inner outlines being separated from each other by an elongated metal strip screening, in a direction normal to said anode surface, the part of the cathode surface which is most adjacent thereto, said cathode screen being closely surrounded by said envelope at least at its edge adjacent thereto and being open in the opposite direction, the cylindrical part of said screen extending, in the same direction, about as far as the cathode.

MINNE KU'IPERS. 

